The present invention generally relates to a radio telephone system and, more particularly, to an emergency speech control in a mobile radio telephone system coupled to a public services telephone network.
Conventionally, a radio telephone system is known in which a communication path between a subscriber accommodated in a public services telephone network (hereafter simply referred to as a PSTN) and a mobile subscriber is established based on a multi-channel access procedure.
Referring to FIG. 1, there is illustrated a conventional radio telephone system which employs a multi-channel access procedure. The illustrated radio telephone system comprises a PSTN and a convenient radio phone system (hereafter simply referred to as a CRP) system. The CRP system is composed of a line control device 1, a base station radio device 2, and an antenna duplexer 3. The base station radio device 2 radio-communicates with a mobile station ML through the antenna duplexer 3 and an antenna 3-1 or 3-2 coupled
thereto. In the illustrated system, m&lt;&lt;P is satisfied where m is the number of channels for use in speech communication, and P is the number of the mobile stations (subscribers) ML. The number m of channels is equal to the number of radio channels.
The line control device 1 includes a main controller 10, a control channel controller 10-0, m speech channel controllers 10-l-10-m, and a line exchanger 11. There are m speech extension lines 12 between the line exchanger 11 and the speech channel controllers 10-l-10-m. The line exchanger 11 exchanges m speech extension lines 12 with N normal speech office lines 13 and n emergency speech office lines 14 (n is an integer). The normal speech office lines 13 and the emergency speech office lines 14 are connected to a subscriber exchanger 4 of the PSTN. The subscriber exchanger 4 accommodates a plurality of subscribers (not shown). Further the exchanger 4 accommodates a plurality of emergency call receiving devices 5 for use in emergency speech communication.
When the illustrated mobile subscriber ML requests a speech communication with a PSTN subscriber, the base station radio device 2 distinguishes a speech request from the illustrated mobile subscriber ML from other communications, and notifies the control channel controller 10-0 of this speech request. The communication channel controller 10-0 temporarily holds the present speech request, and then notifies the main controller 10 of the same. The main controller 10 supervises the m speech channels. When any one of the speech channels becomes available, the main controller 10 allocates the available speech channel to the speech request on the basis of a predetermined priority of speech requests. That is, when the speech request from the illustrated mobile subscriber ML has the highest priority among other competing speech requests, the available speech channel is allocated to the speech request from the illustrated mobile subscriber ML. Then a communication path between the called PSTN subscriber and the mobile subscriber ML is established through the exchangers 11 and 4.
However, a conventional radio speech system such as the above-mentioned system has the following disadvantages. When the mobile subscriber ML requests an emergency speech by dialing a specific dial number ("110" or "119" in Japan), a speech channel between available one of the emergency call receiving devices 5 and the mobile subscriber ML is established. Then the established speech channel is held without being released for a while even after the emergency speech is finished. This is due to the fact that in some cases the operator of the emergency call receiving device 5 wishes to inquire of the mobile subscriber ML something. In the speech channel holding state after finishing the emergency speech, the established speech channel cannot be used for another speech processing. For this reason, the efficiency in using m speech channels 12 is not high. Further, after releasing the established speech path from the holding state, even if the operator of the emergency call receiving device 5 wishes to communicate with the mobile subscriber ML which requested the emergency speech, the operator cannot have access to the mobile subscriber ML. An analogous problem occurs when the base station radio device 2 cannot receive a radio signal from the mobile subscriber ML for a fixed time, when the mobile subscriber ML is in a tunnel, for example. In this case, the established speech path is released and thus the operator of the emergency call receiving device 5 cannot access the mobile subscriber ML.